Precoding/Beamforming (BF) is common technology in an existing Time Division Duplexing (TDD) system in radio communication. As BF in a broader sense, Precoding allows to improve receiving performance of a receiving end through signal weighting at a sending end. Due to channel reciprocity in a TDD system, an Evolved NodeB (eNB) may determine a downlink channel by estimating an uplink channel, and improve receiving-side demodulation performance through the eNB-side Precoding, thereby enhancing system capacity and spectrum utilization.
For example, in a Time Division Dulpexing Long Term Evolution (TDD-LTE) system, there may be two Precoding modes as follows.
In the first Precoding mode, downlink Precoding may be performed according to an uplink channel weight using reciprocity of a TDD system. Given complexity and feasibility of a real system and real equipment, a grain size of a Resource Block (RB) is adopted for a BF weight, where a separate BF weight vector may be adopted for each RB.
In the second Precoding mode, when a Precoding Matrix Indicator-Rank Indicator (PMI-RI) report is enabled, Precoding may be performed by feedback using a default grain size of a Physical Resource Block (PRB) bundling. In an existing standard, a
PRB bundling, in general of 1 RB to 3 RBs in size, is referred to as one Precoding Resource block Group (PRG).
There is no constraint in the first Precoding mode in the existing standard. An eNB may perform Precoding using a grain size of an RB/RB group. The number of RBs in the RB group may be decided by the eNB per se. A UE-default grain size for channel estimation may be of a single RB. An inter-RB/RB group channel phase may no longer be continuous, but become fragmented. In the second Precoding mode, identical Precoding weights may be used for consecutive RBs within a PRG, although an inter-PRG phase may still be discontinuous and fragmented. A UE may perform channel estimation according to the grain size of a PRG, in which case identical weights may be used for the RBs in a PRG, lowering a forming gain to an extent, which is disadvantageous for a TDD system. Both modes may lead to a discontinuous frequency-domain equivalent channel received by a receiving end even with continuous frequency-domain physical resource scheduling by a UE. The UE can only perform channel estimation and demodulation according to a default minimal grain size (RB/PRG).
As the channel phase is originally continuous among RBs in the frequency domain, a channel estimation operation for a downlink Demodulation Reference Signal (DMRS)/pilot as follows shall not be performed due to the existence of phase discontinuity:
1) noise reduction by time-domain/frequency-domain joint channel estimation based on pilots on consecutive RBs; and
2) interpolation based on channel estimation in the time domain/frequency domain, which will lead to a larger error in channel estimation, impacting Precoding performance of the TDD system.
It may be seen that with existing implementation, UE-side channel estimation can be performed only under a relatively small grain size, leading to limited accuracy in channel estimation and poor demodulation performance, thereby impacting overall system performance and spectrum efficiency.